This invention relates to wafer polishing and more particularly to estimating wafer-to wafer thickness variation.
In semiconductor fabrication wafers, such as silicon wafers, after undergoing the pattern processes of forming products such as electronic devices, etc. thereon are coated by a layer of glass or oxide that is on the active layer. Chemical-mechanical polishing (CMP) is widely used as a process for achieving global planarization in semiconductor manufacturing. See G. Shinn, V. Korthuis, A., Wilson, G. Grover, and S. Fang, xe2x80x9cChemical-mechanical polish,xe2x80x9d in Handbook of Semiconductor Manufacturing Technology, ch. 15, pp.415-460, NY: Marcel Dekker, Y, Nishi and R. Doering ed., 2000. The result of the pattern on the wafers makes the polishing rate nonlinear. The hills and valleys resulting from the products under the glass oxide make for the nonlinear polishing.
CMP processes can be categorized into two classes for control purposes: (i) endpointed, and (ii) non-endpointed. In case of endpointed processes, the polish usually involves removal of the film being polished until one hits a stopping layer. Examples of this type of polish include tungsten, STI and copper (damascene) CMP. The endpoint in these cases depends on the difference in the physical properties of the film being polished vs. the stopping layer. Properties commonly used are reflectivity and friction. In contrast to these, non-endpointed processes involve targeting the polish to leave behind a film of a specific thickness. Examples include PMD, ILD and FSG CMP. Typically these processes have proven harder to endpoint in volume production. It is the control of these processes that is the focus of this application. Henceforth, CMP will be used to explicitly refer to such non-endpointed processes.
A key parameter in the control of non-endpointed processes is the blanket polish rate. These blanket (qual) rates are determined using wafers that are not patterned placed on the pad and polished. They are called pilots. The rate of removal of these pilot wafers is linear. This rate of the pilot wafers is the reference rate to which pattern dependent product polish rates are compared. The role of this was highlighted in N. S. Patel, G. A. Miller, C. Guinn, A,. Sanchez, and S., T. Jenkins, xe2x80x9cDevice dependent control of chemical-mechanical polishing of dielectric films,xe2x80x9d IEEE Transactions on Semiconductor Manufacturing, vol. 13, no. 3, pp. 331-343, 2000. This article of Patel et al. reports a state of the art control scheme for controlling these processes based on metrology feedback. Metrology is the measurement of the wafer before and after the polishing. It measures what is left. This is measured with a metrology tool to determine if there is a problem on a lot of wafers. The scheme in Patel et al, cited above, attempts to minimize performance sensitivity to qual wafer frequency, and hence blanket rate samples. However, blanket rate sampling is a prerequisite for any CMP control scheme, since without these samples one loses all observability to the parameters being estimated for control. Applied Materials (AMAT) has proposed interferometry for endpointing, and estimation of blanket rates for such processes on their Mirra polishers. See Birong et al. U.S. Pat. No. 5,964,643. This patent is incorporated herein by reference. However, their algorithms have proven ineffective in both these areas. It is now recognized that reliably endpointing such processes in the presence of production disturbances and shortening polish times is infeasible. Issues lie with varying incoming material thickness off multiple deposition chambers that trigger false endpoints and the quality of the sensor signal (which is viewing the wafer through the slurry) that often results in missed endpoints. On the other hand, estimation of blanket rates is a feasible proposition, however; AMATs algorithm works only on blanket wafers, and is unable to predict blanket rates off product polish which is the case of interest.
In accordance with an embodiment of the present invention an estimation of wafer-to-wafer variation thickness for product wafers includes sensing sample signals representing polishing trace from product wafers near the end of polishing period from at least two product wafers; estimating the value of the phase of the first and second wafers using polish data near the end of the polish period; and calculating the difference in final thickness using the phase difference.